If you fill the tranny through the rear plug then all that 90wt has to pass through the ball bearing to get to the gearbox. Do your tranny a favor...
Fill according to the specs. 90wt Mineral oil in the main fill port and then add some to the rear port, or through the speedometer port. Then you will have properly lubricated the gearbox and the rear bushing/cross joint.

Also a good idea, when you check the lube level in the trans and need to add some, check the rear end level and see if it is higher than expected, a sign of oakie bushing seal failure.

~~ Jethro
1954 3100
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1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020